The invention relates to hollow airfoils for gas turbines and in particular to a tip squealer and a cooling construction for the tip squealer.
The airfoils in gas turbines comprising a pressure side and a suction side which extend from the root to the tip are typically provided with a tip portion. This tip portion protects the airfoil from damage caused by contact with the turbine casing. It consists of a tip cap between the radial ends of the pressure and suction sidewalls and a tip squealer extending radially away from the tip cap along the pressure and suction sidewalls of the airfoil. During operation of the gas turbine the airfoils must withstand very high temperatures. In order to prevent damage due to the high gas temperature, which would shorten the airfoil lifetime, the airfoils are provided with a cooling construction for cooling fluid to flow through and cool the airfoil by various physical means. Between the pressure side and suction sidewalls is a hollow space for cooling fluid, typically air bled from the compressor, to flow through and convectively cool the sidewalls. However, in the region of the tip portion cooling is especially critical as the tip squealer is typically of small thickness and particularly susceptible to high temperature oxidation and other damages due to overheating.
A typical cooling construction for the tip portion is described in EP 0 816 636. A tip squealer extends radially from a tip cap and along the pressure and suction sidewalls of the airfoil. The tip squealer has straight sidewalls and tip crowns of rectangular shape on both the pressure and suction sides. First exit passages for the cooling fluid lead from the hollow space radially through the tip cap to the tip cavity which is enclosed on its sides by the sidewalls of the tip squealer. The cooling fluid flows into the tip cavity and over the suction tip crown, cools that portion convectively, and finally blends into the leakage flow. Second exit passages lead from the hollow space to the pressure side of the airfoil, their axes being oriented at an angle to the radial direction. Cooling fluid flows from the hollow space to the pressure side and passes from there over the pressure side tip crown and through the tip cavity and finally blends into the leakage flow. This type of cooling construction has the disadvantage that in the tip cavity, and in particular along the inside edges of the tip squealer, the cooling fluid can form vortices, which reduce the cooling efficiency. The reduced cooling efficiency results in an increase in the amount of cooling fluid necessary for the cooling.
U.S. Pat. No. 5,183,385 discloses a further cooling construction for the tip portion of a gas turbine airfoil. It comprises a tip squealer with a rectangular cross-sectional shape similar to the construction described above. The cooling passages from the hollow space lead radially through the tip cap into the tip cavity. They have, according to the FIGS. 7-10 of the disclosure, a first straight section and near the surface of the tip cap a funnel-shaped diffused section with a rectangular cross-section such that the outer hole portion defines a rectangular trapezoid. The particular shape provides an expansion of the cooling flow parallel to the squealer surface.
U.S. Pat. No. 5,738,491 describes a further type of cooling construction for an airfoil with a rectangular tip squealer based on convective and conductive cooling. A thermal conductor is fixedly joined to the tip squealer extending radially to the tip cap. The cooling fluid flowing within the hollow space radially inward from the tip cap then removes heat conducted to the tip cap. In a special embodiment the tip cavity is provided with several ribs, chordally spaced and extending between the tip squealer on the pressure side and the tip squealer on the suction side.
In view of the above-noted deficiencies in the prior art, the invention is directed to a tip squealer cooling construction for an airfoil in a gas turbine that yields an improved cooling efficiency about the tip squealer of the airfoil compared to the cooling constructions of the prior art.
An airfoil for a gas turbine with a pressure side and a suction side comprises a pressure sidewall and a suction sidewall extending from the root to the tip of the airfoil. The tip portion of the airfoil comprises a tip cap and a tip squealer. The tip cap forms the radial end surface of the airfoil while the tip squealer is intended to protect the airfoil tip from damage due to contact with the gas turbine casing about the airfoils. The tip squealer extends radially from the pressure sidewall to a pressure side tip crown and from the suction sidewall to a suction side tip crown. It extends along the edge of the tip cap on the pressure and suction sides of the airfoil. The tip cap and tip squealer define a tip cavity or tip pocket.
Within the airfoil a hollow space for cooling fluid to flow is defined by the inner surfaces of the pressure and suction sidewalls and the inner surface of the tip cap. Several exit passages for cooling fluid are directed from the hollow space within the airfoil to the pressure side of the airfoil and several further exit passages for cooling fluid lead from the hollow space through the tip cap to the tip cavity.
According to the invention the tip squealer has a radial cross-section comprising a smooth contour.
The smooth contour of the tip squealer extends from the crown of the tip squealer on the pressure side, into the tip cavity, along the tip cavity and to the crown of the tip squealer on the suction side. The contour comprises one or more curved sections, or several straight sections, or one or more curved as well as straight sections. In particular, the contour of the tip squealer has no abrupt changes in direction. That is, the difference in radius of curvature of the several curved sections and the differences in incline between the straight sections are small.
The cooling fluid that passes through the exit passages on the pressure side flows around the pressure side tip crown and into the tip cavity, along the contoured cavity surface and on to the suction side tip crown where it blends into the leakage flow of the gas turbine.
As a result of the smooth contour, the exit passage extending from the hollow space through the tip cap to the tip cavity is positioned close to the hot gas wall on the suction side of the airfoil. The cooling fluid passes close to the tip crown on the suction side and thus close to the hot gas surface. This enables a near wall cooling, which removes the heat load near the top portion of the suction side. In comparison, in a conventional tip squealer the exit hole of the cooling passage is placed on the surface of the tip cap and much farther away from the tip crown.
The smooth contour allows an even flow of the cooling fluid about the tip crowns and within the tip cavity. The cooling fluid flowing over the smooth contour experiences no abrupt changes in flow direction, as there are no sharp corners or other abrupt changes in incline. In particular, the smooth contour avoids the formation of vortices. The resulting smooth flow of the cooling fluid enables an enhanced film cooling of the tip cap surface and the tip squealer. This results in an increased cooling effectiveness, which in turn reduces the necessary amount of cooling fluid.
The heat load that is transmitted from the tip portion into the airfoil is proportional to the surface area of the airfoil tip portion, also referred to as the hot gas side surface. The smoothly contoured tip squealer according to the invention has a smaller hot gas side surface compared to a conventional tip squealer with a rectangular contour. Therefore, from the smaller hot gas side surface of the airfoil according the invention a smaller heat load needs to be transmitted into the airfoil, and in turn the necessary amount of cooling fluid is reduced.
Finally, the tip squealer with a smooth contour according to the invention yields an increased tip section fin efficiency, which is the ability to transmit the heat load away from the tip squealer. The tip squealer extends radially away from the airfoil in the manner of fins and conducts the heat load away from the tip crowns through the base area of the fins to the primary airfoil cooling passages or the hollow space within the airfoil. The tip squealer with a smooth contour has an increased base area compared to a rectangular tip squealer and therefore conducts heat away from the tip crowns more efficiently.
In a particular embodiment of the invention the tip squealer comprises within the tip cavity one or more curved portions, or one or more straight portions, or one or more straight and curved portions. Again the incline angles of the straight portions and the-radii of curvature of the curved portions are chosen such that there are no abrupt changes in direction of a cooling fluid flowing over the surface of the tip cavity and about the squealer tip crowns.
In a particular and preferred embodiment of the invention the contour of the tip squealer comprises within the tip cavity two curved portions and one straight portion between the pressure side tip crown and the center of the tip cavity. The first curved portion extends from the pressure side tip crown toward the center of the tip cap and preferably has a radius of curvature less than 0.03 inch. The second curved portion extends from the first portion toward the center of the tip cap and has a radius of curvature greater than the height of the squealer and preferably greater than 0.4 inch. The straight portion extends from the second curved portion to the center of the tip cap and is at a 3xc2x0 to 45xc2x0 incline angle relative to the center line of the tip cap.
In a further preferred embodiment of the invention the contour of the tip squealer comprises within the tip cavity a second straight portion extending from the center of the tip cap to the inner edge of the suction side tip crown. This second straight portion is at a 15xc2x0 to 45xc2x0 incline angle relative to the center line of the tip cap.
In a further preferred embodiment of the invention the exit passages that extend from the hollow space to the pressure side of the airfoil each have a passage axis that is oriented at an angle with respect to the radial direction. The radial direction is defined as the radially outward direction of the inner surface of the pressure sidewall. The passage axis is furthermore oriented at an angle with respect to the streamwise direction, which is the direction along the hot gas flow from the leading edge to the trailing edge of the airfoil.
In a particular embodiment of the invention the axis of the exit passage extending to the pressure side of the airfoil is directed at an angle with respect to the radial direction that is in the range from 15xc2x0 to 65xc2x0, preferably in the range from 20xc2x0 to 35xc2x0 directed away from the pressure side tip crown, and at an angle with respect to the streamwise direction that is in the range from 30xc2x0 to 90xc2x0, preferably in the range from 45xc2x0 to 90xc2x0.
In a further particular embodiment of the invention the exit passages extending from the hollow space through the tip cap to the tip cavity each have a passage axis that is oriented at an angle with respect to the radial direction as well as at an angle with respect to the streamwise direction.
In a preferred embodiment of the invention the angle of the axes of the exit passages with respect to the radial direction is in a range from 0xc2x0 to 45xc2x0, preferably from 20xc2x0 to 30xc2x0, and directed toward the suction side tip crown. The angle with respect to the streamwise direction is in a range from 35xc2x0 to 90xc2x0, preferably from 35xc2x0 to 55xc2x0.
These particular orientations of the axes of the exit passages direct the cooling fluid flow more smoothly onto the tip squealer such that the cooling fluid flows onto the tip squealer with no great changes of direction. This measure contributes to a further increase in film cooling effectiveness.
In a further embodiment of the invention each of the exit passages leading to the pressure side have a diffused shape either over the entire length of the exit passage or at least over the end portion of the exit passage leading to the exit port. In the latter case the exit passage has a cylindrical shape beginning at the hollow space of the airfoil and extending into a part of the exit passage length and a diffused shape extending from the cylindrical part to the exit port of the passage. The cylindrically shaped portion of the exit passage is intended to meter or control the cooling flow through the passage.
Furthermore, the diffusion of the exit passage is either on all sides of the passage axis or only to one side of the passage axis. In the latter case, the diffusion is directed toward the pressure side tip crown of the squealer. The exit passage then has a cross-section perpendicular to the cooling fluid flow direction that is partially circular and partially oval.
In a further embodiment of the invention the same characteristics apply to the exit passages leading from the hollow space to the tip cavity. They comprise a diffused shape directed toward the suction side tip crown. Again the diffused shape is either over the entire length of the exit passage or at least over the end portion of the exit passage leading to the exit port of the passage. In the latter case the exit passage has a cylindrical shape beginning at the hollow space of the airfoil and extending into a part of the exit passage length and a diffused shape extending from the cylindrical part to the exit port of the passage.
Furthermore, the diffusion is either to all sides of the passage axis or only to one side of the passage axis. In the latter case, the diffusion is directed toward the suction side tip crown of the squealer. The exit passage then has a cross-section perpendicular to the cooling fluid flow direction that is partially circular and partially oval.
The diffusion of the exit passage is intended to spread out the cooling fluid as it approaches the exit port of the exit passage and lower its exit velocity as it flows onto the tip squealer. This yields a further improvement of the film cooling effectiveness as a greater amount of the cooling fluid remains close to the surface of the tip squealer.
In a particular embodiment of the invention the sidewalls of the exit passages extending to the pressure sidewall are oriented at an angle with respect to the exit passage axis that is in the range from 7xc2x0 to 12xc2x0 and directed toward the pressure side tip crown.
In a further particular embodiment the sidewalls of the exit passage extending from the hollow space to the tip cavity has sidewalls that are oriented at an angle with respect to the exit passage axis in the range from 7xc2x0 to 12xc2x0 and directed toward the suction side tip crown.
In a further embodiment the exit passages from the hollow space to the tip squealer, both the exit passages leading to the pressure side as well as to the tip cavity, have sidewalls that are diffused at an angle with respect to the passage axis and directed toward the streamwise direction.
This effects a wider flow from the exit passage onto the tip squealer surface and further improves the film cooling.